As described in Non-Patent Documents 1 to 3, in ultra long-haul large-capacity transmission, notably an optical submarine cable system, for example, in order to maximize an optical signal-to-noise ratio in the system, optical fibers with a low transmission loss and large effective area (Aeff) (Low-loss large-Aeff optical fibers) have been used. At present, large Aeff optical fibers having an Aeff of 130 to 150 μm2 at a wavelength of 1.55 μm have been adopted to the optical submarine cable system. However, in repeaters of the system, single-mode optical fibers conforming to ITU-T G.652 or G.654C have been commonly used as feedthroughs.
A typical connection loss caused by fusion-splicing between the large Aeff optical fiber and the single-mode optical fiber is 0.3 dB per connection at a wavelength 1550 nm. As described in Non-Patent Document 4, a typical span loss in a transmission system having a capacity distance product exceeding 500 Pb/s×km is 10 dB. The above-described connection loss is too high to be neglected as compared with this span loss. For this reason, it is desirable to reduce the connection loss between the large Aeff optical fiber and the single-mode optical fiber.
One of the causes of the connection loss between the large Aeff optical fiber and the single-mode optical fiber is that a difference in mode field diameter (MFD) between the large Aeff optical fiber and the single-mode optical fiber prevents all optical power from being coupled to a fundamental mode in a transition section extending across the two optical fibers (a section where the MFD changes along the longitudinal direction). In Non-Patent Document 1, described is that a large Aeff optical fiber having a double core (ring type core) structure in which a second core located outside a first core has a higher refractive index can make the MFD small relative to the same Aeff as compared with a large Aeff optical fiber having a core (step type core) structure that has a conventional step type refractive index distribution. Therefore, the use of an optical fiber having a ring type core as the large Aeff optical fiber rather than an optical fiber having a step type core makes it possible to reduce an MFD mismatch (makes it possible to reduce the connection loss). However, a theoretically calculated connection loss between a single-mode optical fiber with an Aeff of 83 μm2 at a wavelength of 1.55 μm and a large Aeff optical fiber having a ring type core with an Aeff of 148 μm2 at the wavelength of 1.55 μm remains high at 0.22 dB.
Examples of a method for further reducing the connection loss between ends of two optical fibers that are different from each other in MFD include a bridge connection for interposing and connecting an optical fiber having an intermediate Aeff, a taper connection for physically tapering a connection point, and a core diffusion connection (thermally expanded core (TEC) connection) for heating a connection point to enlarge a core.
In Patent Document 1, disclosed is a method of bridge connection using an ultra-short bridge fiber. The bridge connection causes the number of connection points to increase, which may make a system complicated. Further, in Non-Patent Document 3, described is a loss reduction method using taper connection. Note that the taper connection causes the connection point to be tapered, which may lead to a reduction in the mechanical strength.
The TEC connection does not have a risk arising from the bridge connection or the taper connection and is the most practical connection method for an optical submarine cable system and the like. The TEC connection is described in Non-Patent Document 5. Forming a tapered MFD (a state in which the MFD continuously increases or decreases along the longitudinal direction) around the connection point reduces or eliminates the MFD mismatch at the connection point. The tapered IVIED is generally realized by diffusing, by heat, a dopant doped to a core of an optical fiber to enlarge the MFD.